Papers by Keyword: Mechanical Instability

Abstract: By using the mobile property of silicone oil during deposition, characteristic wedge-shaped chromium (Cr) films have been successfully prepared on glass substrates by direct current magnetron sputtering, and their mechanical instability is investigated. The experiment shows that a disordered network composed of straight-sided buckles, which is due to relief of large compressive stress after deposition, can be widely observed in these films. The size of the buckles increases linearly with the film thickness. In the vicinity of spreading front of the silicone oil, parallel straight-sided buckles perpendicular to the front and bifurcation morphologies can be observed. The influence of the edge effect on the buckle morphologies is also discussed in this paper.

Abstract: The occurrence of various instabilities at very high speed is well known to occur in brittle fracture and significant advances have recently been obtained in the understanding of their origin. On the other hand, low speed brittle crack propagation under pure tension loading (mode I) is usually thought to yield smooth crack surfaces. The experimental investigation reported here questions this statement. Steady cracks were driven in brittle glassy polymers (PolyMethyl Methacrylate - PMMA) using a wedge-splitting geometry over a wide range of low velocities (10^-9-10^-1 m/s). Three distinct patterns can be observed on the post-mortem fracture surfaces as crack velocity decreases: perfectly smooth at the highest speed, regularly fragmented at intermediate speed and macroscopically rough at the lowest speed. The transition between the two latter is reminiscent of chaotic transition. ^ahattali.lamine@gmail.com, ^bjonathan.bares@cea.fr, ^cponson@caltech.edu, ^ddaniel.bonamy@cea.fr,

Abstract: The mechanical stability of a material is a fundamental issue in strength of atomic systems. Although the criterion of the mechanical stability of homogeneous structures such as perfect crystals have been successfully investigated so far, the criterion has not been able to be precisely evaluated in the cases of non-uniform deformations or bodies of inhomogeneous atomic structures. Now we present an instability criterion of an arbitrary atomic structure based on the energy balance of the whole system. This method gives the mathematically rigorous condition for the onset of an unstable deformation in any inhomogeneous atomic system. Furthermore, the method can be applied to any type of potential field, which means that ab initio evaluations of the mechanical instability of inhomogeneous structure under non-uniform deformation will be possible. The validity of the method is clarified by the application to tension of a cracked body. The onsets of unstable deformations and their deformation modes are precisely evaluated by the method.